The multicellular eukaryotic organism is made up of many different cell types. Studies in modern biology have shown that specific genes are expressed in one cell type and not in another.
It is known that particular types of cell can be taken from an animal and grown in tissue culture for a short period. However, these primary cells have a limited life span in culture. Methods are available which allow cells to grow indefinitely in culture. For example, cells can be derived from tumors which have cell-type specific properties and grow continuously in culture. It is now known, however, that tumor cells differ from their normal counterparts because they have undergone heritable and specific changes at the DNA level which change the growth state of these cells. The genes which alter the growth state of cells when they are altered, either in their regulatory or effector properties, are known as oncogenes.
As a result, although tumor-derived cells have proven to be very useful, particularly because they make long-term growth of cells in culture possible, because they differ from corresponding normal cells in many ways, they have limited use in many applications. It would be useful to have a means of producing or growing quantities of normal cells with defined characteristics.